Orientational and Crystalline Order of Copper–Phthalocyanine Films on Gold: the Role of Substrate Roughness and Cleanliness

Abstract

Although metal phthalocyanines are widely used in optoelectronic devices, e.g., as hole-transport and electron-blocking layers, or as UV-stable dyes, their multilayer growth on metal substrates has surprisingly not been studied very systematically. Even for CuPc, one of the most widely studied representatives of phthalocyanines, contradictory structures are reported for films grown on gold, a common electrode material, suggesting that the influence of actual substrate surface properties on film growth has not been sufficiently considered. In this study, we analyze the growth of CuPc films on gold substrates for thicknesses ranging from the initial seed layer to thick multilayers (50 nm) by combining near-edge X-ray absorption spectroscopy with atomic force microscopy and X-ray diffraction. To study the influence of surface roughness, we compare the formation of CuPc films on well-ordered Au(111) and sputter-deposited polycrystalline gold substrates and also investigate the influence of surface contamination by exposing these gold surfaces to air before film growth. While on clean gold substrates, CuPc molecules exclusively adopt a recumbent orientation and form (112̅)-oriented films, they also grow in an upright orientation on contaminated gold surfaces. On Au(111), this leads to the coexistence of (112̅)- and (100)-oriented regions, whereas only (100)-oriented films are formed on contaminated polycrystalline gold. Remarkably, the (112̅)-oriented films consist of extended but isolated crystalline islands, resulting in large overall roughness, whereas the (100)-oriented films consist of rather small domains but have significantly lower film roughness.

Metal phthalocyanines: thin-film formation, microstructure, and physical properties

“…Areas of high surface roughness decrease the barrier for heterogeneous nucleation by decreasing the diffusion distance of molecules. 16,[42][43][44][45][46][47] This results in small grain formation, enhanced defects, and oen a different molecular orientation relative to the substrate as exhibited by thin-lms of CuPc (Fig. 6i).…”

“…6i). [42][43][44] Altering the surface chemistry of the substrate through self-assembled monolayers (SAMs) is a common strategy to inuence the morphology and crystallinity of small molecule thin-lms. SAMs are highly ordered 2D structures consisting of a head group, terminal group, and linker.…”

Interface Properties of CoPc on Nanographene-Covered Au(111) and the Influence of Annealing

“…48 Furthermore, it is known that the substrate roughness and cleanliness may affect distinctly the arrangement order of organic films on gold. 51 We note that data of electronic interface properties indicate that there is no noticeable molecular exchange between HBC molecules of the first ML and CoPc (see below). Thus, the presence of HBC molecules in the top layer appears unlikely.…”

Mapping the Chiroptical Properties of Local Domains in Thin Films of Chiral Silicon Phthalocyanines by CD Imaging

“…A 10-mL microwave tube was charged with silicon phthalocyanine dichloride (PcSi(Cl) 2 ; 7.2 mg, 0.012 mmoles, 1 equiv), (S)-3-phenyl-2-(1,3,6,8-tetraoxo-7-((S)-2-phenylpropyl)-3,6,7,8tetrahydrobenzo[lmn] [3,8]phenanthrolin-2(1H)-yl)propanoic acid (S,S-NDI) (33 mg, 0.62 mmoles, 5 equiv), N,N-diisopropylethylamine (0.5 mL) and toluene (5 mL). The reaction mixture was heated under microwave irradiation for 14 h at 155 • C. The blue residue obtained after solvent evaporation under reduced pressure was redissolved in ethyl acetate (50 mL).…”

“…Among these, metallophthalocyanines (Pcs) are of particular interest due to their high thermal and chemical stabilities [2][3][4][5]. Pcs are semiconductors [2,3,6,7] with excellent optical and electrical properties, being widely used for write-once read-many materials, organic photovoltaics (OPVs), organic light emitting diodes (OLEDs), organic thin-film transistors (OTFTs), non-linear optical devices, etc. [2,4,[6][7][8].…”